1. Field of the Invention
The present invention relates to a system and method to increase channel utilization in a wireless communication network using a multichannel medium access control (MAC) protocol. The messages are sent directly on the reservation channel without any request-to-send/clear-to-send (RTS/CTS) handshake. Each message can further be assigned a special type indicator allowing it to be easily distinguished from the RTS/CTS message.
2. Description of the Related Art
Wireless communications networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communication signals may include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets. As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base node to communicate simultaneously with several mobile nodes within its coverage area.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed for use by the military. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in conventional ad-hoc networks, further enable mobile nodes to access a fixed network and thus communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. Pat. No. 7,072,650 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, granted on Jul. 4, 2006, in U.S. Pat. No. 6,807,165 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, granted on Oct. 19, 2004, and in U.S. Pat. No. 6,873,839 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, granted on Mar. 29, 2005, the entire content of each being incorporated herein by reference.
Communication between nodes in such networks, however, is often subject to collisions. One collision avoidance technique is carrier sense multiple access protocol (CSMA). In this technique, all nodes are forced to wait for a random number of timeslots and then sense the activity of the medium before starting a transmission. If the medium is sensed to be busy, the node freezes its timer until the medium becomes free again, thereby reducing the possibility of two nodes starting to send messages simultaneously. Therefore, as known to those skilled in the art, the range of the random delay, or the contention window, is set to vary with the load, and ARQ is used to finish the successful transmission process. After sending a packet, the sender waits for the “acknowledgement” (ACK) from the receiver. If ACK is not received within the specified time, the sender assumes that a collision has happened, and a retransmission is needed. In the case of a collision, the random delay range is increased progressively until a successful transmission occurs and the delay range is reset to the minimal value. In this case, the CSMA protocol handles ARQ by repeating the whole request to send/clear to send channel access sequence. Large delays are thus incurred due to retransmissions.
One problem associated with CSMA is that “carrier sense” only can detect the interference around the sender. However, of greater concern is interference at the receiver. In order to solve this problem, Phil Karn presented the Multiple Access with Collision Avoidance (MACA) protocol as described in the article entitled “MACA-A New Channel Access Method For Packet Radio”, the entire content of which is incorporated herein by reference. In MACA, a node requiring to transmit data to a receiver first sends a request-to-send (RTS) packet to the receiver to clear the sender's area. Upon receiving RTS successfully, the receiver responds with a clear-to-send (CTS) packet to clear the receiver's area. The RTS/CTS carries the information, which includes source/destination addresses, transmission duration, and so forth, for the intended transmission. Therefore, all nodes overhearing the RTS/CTS will hold their transmission long enough to avoid collision.
MACA protocol introduced RTS-CTS-DATA process to overcome the problem in CSMA. In order to accelerate the confirmation of the correct message transmission, the link layer ACK is introduced in MACAW proposed in “MACAW: A Media Access Protocol for Wireless LAN's” by V. Bharghavan et al., the entire content of which is incorporated herein by reference. One data transmission in MACAW includes RTS-CTS-DATA-ACK exchanges between the sender and the receiver. A variant of MACAW is used in IEEE 802.11 and is known as CSMA/CA. The multichannel MAC design is an extension of MACA/MACAW/CSMA/CA protocols. One channel is reserved for the transmission of control packets such as RTS, CTS, and other channels are used for data packet transmission. The typical process is described below.
In such a process, all nodes in the network keep listening to the reservation channel. Before data packet transmission, RTS/CTS are exchanged between the source and destination via the reservation channel. In RTS/CTS, besides the source/destination addresses and transmission duration information, the information of selected data channel is also carried. Once the exchange of RTS/CTS is successful, both sender and receiver tune to the selected data channel to transmit the data packet and ACK. Upon receiving the ACK, both sender and receiver retune back to the reservation channel to prepare for the future transmission. If a collision happens, the backoff process similar to which is in CSMA will be performed to resolve the collision.
In the multichannel MACA/MACAW and CSMA/CA protocols, the control packet overhead and the tuning delays between reservation channel and data channels are very high which reduces the channel utilization. This problem is more serious when the data packet size is very small. However, in order to coordinate nodes in Ad-Hoc networks, it is very important to distribute node information among neighbor nodes in the form of short Neighbor Advertisement (NA) or short “Hello” message. Accordingly, a need exists for minimizing the transmission overhead for small size packet in the multichannel MACA/MACAW or CSMA/CA protocol.